LEDs are typically driven using a DC-DC converter. The converter accepts a DC input voltage and provides a DC output voltage.
For many applications, DC-DC converters are configured to provide a regulated DC output voltage to a load based on an unregulated DC input voltage. A DC-DC converter may be employed to transform an unregulated voltage provided by any of a variety of DC power sources to a more appropriate regulated voltage for driving a given load. The unregulated DC input voltage is typically derived from a mains AC power source which is rectified and filtered by a bridge rectifier/filter circuit arrangement.
FIG. 1 shows a circuit diagram of a conventional step-down DC-DC converter 50 configured to provide a regulated DC output voltage 32 (Vout) to a load 40, based on a higher unregulated DC input voltage 30 (Vin). The step-down converter of FIG. 1 also is commonly referred to as a “buck” converter. From a functional standpoint, the buck converter of FIG. 1 generally is representative of other types of DC-DC converters.
DC-DC converters like the buck converter of FIG. 1 employ a transistor or equivalent device 20 that is configured to operate as a saturated switch which selectively allows energy to be stored in an energy storage device 22. The energy storage device 22 is shown as an inductor L in FIG. 1.
Although FIG. 1 illustrates such a transistor switch as a bipolar junction transistor (BJT), field effect transistors (FETs) also may be employed as switches in various DC-DC converter implementations. By virtue of employing such a transistor switch, DC-DC converters also are commonly referred to as “switching regulators” due to their general functionality.
The transistor switch 20 in the circuit of FIG. 1 is operated to periodically apply the unregulated DC input voltage 30 (Vin) across the inductor 22 (L) for relatively short time intervals (in FIG. 1 a single inductor is depicted to schematically represent one or more actual inductors arranged in any of a variety of serial/parallel configurations to provide a desired inductance).
During the intervals in which the transistor switch is “on” or closed and thereby passing the input voltage Vin to the inductor, current flows through the inductor based on the applied voltage and the inductor stores energy in its magnetic field. When the switch is turned “off” or opened so that the DC input voltage is removed from the inductor, the energy stored in the inductor is transferred to a filter capacitor 34 which functions to provide a relatively smooth DC output voltage Vout to the load 40.
When the transistor switch 20 is on, a voltage VL=Vout−Vin is applied across the inductor 22. This applied voltage causes a linearly increasing current IL to flow through the inductor (and to the load and the capacitor) based on the relationship VL=LdIL/dt.
When the transistor switch 20 is turned off, the current IL through the inductor continues to flow in the same direction, with a diode 24 (D1) now conducting to complete the circuit. As long as current is flowing through the diode 24, the voltage VL across the inductor is fixed at Vout−Vdiode, causing the inductor current IL to decrease linearly as energy is provided from the inductor's magnetic field to the capacitor and the load.
FIG. 2 is a diagram illustrating various signal waveforms for the circuit of FIG. 1 during the switching operations described above.
Conventional DC-DC converters may be configured to operate in different modes, commonly referred to as “continuous” mode, “discontinuous” mode or “critical” mode.
In continuous mode operation, the inductor current IL remains above zero during successive switching cycles of the transistor switch. In critical mode, the inductor current starts at zero at the beginning of a given switching cycle and returns to zero at the end of the switching cycle. In discontinuous mode, the inductor current starts at zero at the beginning of a given switching cycle and returns to zero before the end of the switching cycle.
FIG. 3 shows waveforms for the continuous mode assuming no voltage drops across the transistor switch when the switch is on (i.e., conducting) and that there is a negligible voltage drop across the diode D1 while the diode is conducting current. The changes in inductor current over successive cycles are shown in FIG. 3 superimposed on the voltage at the point VX shown in FIG. 1, based on the operation of the transistor switch 20 and the current through the inductor IL for two consecutive switching cycles. The horizontal axis represents time t and a complete switching cycle is represented by the time period T, wherein the transistor switch “on” time is indicated as ton and the switch “off” time is indicated as toff (i.e., T=+ton +toff).
For steady state operation, the inductor current IL at the start and end of a switching cycle is essentially the same, as can be observed in FIG. 3 by the indication Ip. Accordingly, from the relation VL=L dIL/dt, the change of current over one switching cycle is zero, and may be given by:
      dI    L    =      0    =                  1        L            ⁢              (                                            ∫              0                              t                out                                      ⁢                                          (                                                      V                                          i                      ⁢                                                                                          ⁢                      n                                                        -                                      V                    out                                                  )                            ⁢              d              ⁢                                                          ⁢              t                                +                                    ∫                              t                on                            T                        ⁢                                          (                                  -                                      V                    out                                                  )                            ⁢              d              ⁢                                                          ⁢              t                                      )            which simplifies to
                              (                                    V                              i                ⁢                                                                  ⁢                n                                      -                          V              out                                )                ⁢                  t          on                    -                        (                      V            out                    )                ⁢                  (                      T            -                          t              on                                )                      =    0    or                              V          out                          V                      i            ⁢                                                  ⁢            n                              =                                    t            on                    T                =        D              ,  
D is defined as the “duty cycle” of the transistor switch, or the proportion of time per switching cycle that the switch is on and allowing energy to be stored in the inductor. It can be seen that the ratio of the output voltage to the input voltage is proportional to D; namely, by varying the duty cycle D of the switch in the circuit of FIG. 1, the output voltage Vout may be varied with respect to the input voltage Vin but cannot exceed the input voltage, as the maximum duty cycle D is 1.
The conventional buck converter of FIG. 1 is particularly configured to provide to the load 40 a regulated output voltage Vout that is lower than the input voltage Vin. To ensure stability of the output voltage Vout, the buck converter employs a feedback control loop 46 to control the operation of the transistor switch 20. Generally, as indicated in FIG. 1 by connection 47, power for various components of the feedback control loop 46 may be derived from the DC input voltage Vin or alternatively another independent source of power.
A scaled sample voltage Vsample of the DC output voltage Vout is provided as an input to the feedback control loop 46 (via the resistors R2 and R3) and compared by an error amplifier 28 to a reference voltage Vref. The reference voltage is a stable scaled representation of the desired regulated output voltage Vout. The error amplifier 28 generates an error signal 38 based on the comparison of Vsample and Vref and the magnitude of this error signal ultimately controls the operation of the transistor switch 20, which in turn adjusts the output voltage Vout via adjustments to the switch duty cycle. In this manner, the feedback control loop maintains a stable regulated output voltage Vout. In particular, the error signal 38 serves as a control voltage for a pulse width modulator 36 which also receives a pulse stream 42 having a frequency/=1/T provided by an oscillator 26. In conventional DC-DC converters, exemplary frequencies for the pulse stream range from approximately 50 kHz to 100 kHz. The pulse width modulator 36 is configured to use both the pulse stream 42 and the error signal 38 to provide an on/off control signal 44 that controls the duty cycle of the transistor switch 20. In essence, a pulse of the pulse stream 42 acts as a “trigger” to cause the pulse width modulator to turn the transistor switch 20 on, and the error signal 38 determines how long the transistor switch stays on and hence the duty cycle D.
The continuous mode is described above. The “discontinuous” or “burst” mode is used to improve the light-load efficiency of the power converter, to help save energy and extend the battery life of devices. Switching losses can also be reduced by reducing the switching frequency at these light load conditions. When the load current is high, it is preferable to operate the power converter in a fixed frequency continuous mode as this allows a fast transient response, higher efficiency, and a narrower-spread noise spectrum. When the load current is low, the control switch turns on for a few consecutive cycles and stays off until the output voltage drops below the threshold. However, this burst mode operation is in some cases not desirable because a burst mode at light load can cause the switching noise spectrum to spread to a wide range, imposing EMI issues and giving visible flickering when driving an LED.
There is a need to control the transition between different modes. U.S. Pat. No. 7,755,342 discloses a circuit for transitioning between a fixed frequency mode and a discontinuous mode. However, there is an abrupt change in the circuit response at the transition between modes. Furthermore, this approach makes use of the burst mode which can give rise to flickering.